Napoleon, I think.

Well, Napoleon is essentially Empire but further developed. Not much sense in going backwards, but worth a diversion if y'get it in a sale, I suppose.

Explain the simplicity of the game, I still want to get both Medieval and Empire.

Very simplified economy, very few diplomatic options, and things like that.

This is the game for people who want to focus on battles, and you will get plenty of options to fight a battle, dozens of different unit types and all. And fancy graphics.

But the strategic layer of the game lacks depth in almost every aspect.

The perfect game would be to combine the complexity of Paradox strategy and Total War battles :P

Don't get me wrong, I think Total War series are great games, but aimed at different target than myself.

Is this game's battle favors people who are slow and strategic, not people who are simply fast at making units and basses ala StarCraft and Command & Conquer?

Strategic map is turn based, so you don't have to click fast. And during the battle there is an active pause.

And unless things have changed drastically since RTW, you make all your units during the strategic phase, not in the actual battle. You don't really construct anything during the battle itself, it's much more about tactical positioning than a traditional RTS. For instance, a cavalry charge head on against pikemen is pretty much doomed. Send a unit of peasants against them, then charge in your cavalry from behind, and you might rout them immediately.

Yeah, I'm not a fan of Japan history myself, so I'm having a hard time getting into Shogun 2.

What I love about Napoleon: Total War, however, is the drastically different approach to battles when compared to most other TW games: Most of them have a lots of specialized units - there are some which are very versatile, but not all that many. The Napoleonic era warfare pretty much means that you have a lot very versatile units, however a lot of them are a bit more efficient at something else - I find it gives me a lot of tactical flexibility while retaining the need to take the unit type into account as well. And, of course, muskets and canons work a lot differently when compared to swords and arrows, the game reflects that quite nicely.

But yeah, objectively, I'd say Medieval 2 was the best in the series. It also had just about the most epic sieges in videogames - big cities were BIG!

Empire is the one of the best campaign maps but one of the worst at combat. It's the biggest campaign map in a Tw game and the first to have minor factions such as Knights of Malta. Plus government type is as important as religion in this one so another first.

Yeah, especially sieging cities like Constantinople.
You don't build units in the real-time battles. Whatever is in is brought during the initiation of the battle during the turn-based campaign map. You train/hire units in the turn-based map and form a army which you can move around the map. If that army has 5 units of light infantry and 3 units of heavy cavalry, you'll see 5 units of light infantry and 3 units of heavy cavalry in-game. Number of troops in the unit depends on the unit itself, say 100 per unit of light infantry and about 24 cavalry per unit of heavy cavalry.
You can't rush the enemy like you would be traditional RTSes, positioning is important like facing a spear unit directly at the enemy horsemen. If find your flanks and backs towards the enemy, you're in deep shit.

This is a historical battle in the upcoming Rome 2, shld explain the battle mechanics very well:
http://www.youtube.com/watch?v=Kg42DC4EpMY